[He wonders for a second if the other two just have no adults to ask or if they tossed this off onto Sasha, both of which don't sit well with him. But she says she's not sure about it yet, so maybe she's simply being proactive. It's a harmful routine to get into, as far as Will's concerned, so he's quiet for a few seconds before:]

If they can't find anyone else to act as a guardian for them, you can give them my name and tell them to talk to me about it. We'll set it up from there.

[He won't say no, but he won't say yes without 100% certainty and actually talking to them. Seems fair and balanced.]